In general, in order to simultaneously obtain a variable speed operation and high efficiency of a motor, various structures and forms of motors have been proposed. Representatively, there are a variable flux memory motor (VFMM) (hereinafter, called “prior art 1”) and a motor disclosed in Japanese Patent Laid-open No. 2009-112454 (hereinafter, called “prior art 2”).
A rotor of the variable flux memory motor according to the prior art 1 is basically similar with a brushless DC electric motor (BLDC motor) of a spoke type. The motor is a motor that permanent magnets are demagnetized from a narrower part thereof due to a difference in thickness of the permanent magnets when a negative d-axis current flows to a d-axis which is a magnetic flux generation axis of a stator. The motor is operated at a variable speed by demagnetizing and magnetizing the permanent magnets based on the above principle.
The motor according to the prior art 2 is basically similar with an outer-rotor type BLDC motor of a salient pole concentrated winding structure. The motor is characterized in that two kinds of magnets with different coercive forces are embedded in a rotor core in such a way as to be arranged in a circumferential direction in turn to thereby form opposite poles. That is, the rotor core has holes for embedding first magnets and second magnets therein and protrusions formed on an inner face of the rotor core. Accordingly, the motor according to the prior art 2 has several problems in that the rotor core is complicated in structure and manufacturing costs are increased.
In the meantime, the motor according to the prior art 2 includes the first magnets and the second magnets having lower coercive force than the first magnets and is operated in such a way as to increase the magnetic flux of the entire magnets at the time of the laundry process by magnetizing the second magnets but to decrease the magnetic flux of the magnets at the time of the dewatering process more than the laundry process by demagnetizing the second magnets. However, in the case that the coercive force of the second magnets is relatively much lower, in other words, in the case that an increased amount of the entire magnetic flux is almost negligible even though the second magnets are magnetized and operated, in terms of effectiveness in operation, it is advantageous to use the magnetic flux of the first magnets rather than to magnetize the second magnets at the time of the laundry process and to offset the entire magnetic flux by inversedly magnetizing the second magnets at the time of the dewatering process.
Therefore, the inventors of the present invention propose a variable magnetic flux motor using the above-mentioned method.